I'm not going to have time to deep dive an explanation on this, but you're not thinking of the data sets how they should be considered.
Taking an incredibly simple example, let's go with the entire population is 100 and it is composed of only Fighters and Clerics.
There are 70 fighters and 30 clerics. The 70 fighters choose (let's say) among 10 subclasses and (let's say) equally for the example. That means that for those 10 fighters subclasses, they each only compose 7% of the total percentage of individual subclasses.
For the 30 clerics, let's say that 20 of those are Life Domain, and the rest are Knowledge Domain (10). This gives us a final breakdown of:
20% Life Domain, 10% Knowledge Domain, and 10 other subclasses at 7%. Even though there are only 30 total clerics compared to the 70 total fighters, Life Domain still comprises the highest individual percentage.
Of course mileage is going to vary on any of this. The intent is to demonstrate which subclass choices are popular comparatively. If someone chooses cleric, there's a pretty high percentage they're choosing Life, same as with Draconic sorcerer. Fighter is the most popular class overall, but they have a higher number of better-represented subclasses, therefore bringing their individual percentages lower.
It might be more helpful to consider the rankings by class I shared back in April. (This is not updated for current, but we will provide that in the near future. I couldn't figure out how to inline this, so I tried to attach.)